chore(events): ignore the dead coach_description column

[mroderick]

Part of #3019. Sets up the drop of the dead events.coach_description column, following the two-step approach proven in the meetings, workshops and unsubscribed stacks.

coach_description is a text column from the early event-invites feature. Its reads were removed in de44fe17 (2016-06-14), which replaced the coach-specific text in invitation mailers with the general event.description. Nothing reads it today and it is not on any admin form; the production snapshot has 3 non-null values of 260 rows. This PR adds the column to self.ignored_columns on Event and removes the fabricator line, the only write left in the codebase. The second, stacked PR removes the column.

Detail

coach_description appears nowhere else outside db/schema.rb. The legacy-column spec in spec/models/event_spec.rb is transient: the stacked drop PR removes it, same pattern as meetings and workshops.

Review notes

• Nothing risky: mirrors the merged chore(meetings) and chore(workshops) ignore PRs one column over.
• Deliberately not done: the column removal itself — that is the stacked PR, which merges after this one deploys and soaks.

[mroderick]

This has been deployed as part of v794